MavEtJu's Distorted View of the World

We have a ExtremeWare Summit/400 gigabit ethernet switch/router
and a RiverStone RS/3000 fast ethernet switch/router. When using a
single ethernet cable between the two, we often peaked at 80-90%
utilisation and therefor added more ethernet cables and trunked
them into one big virtual ethernet cable.

Both the RS/3000 and Summit/400 have different algorithms for choosing the physical cable the ethernet packet goes over:

The RS/3000 selects on stream: That is the combination of IP address of source and destination and the TCP/UDP port (if it are TCP or UDP packets). If the data is coming from the RS/3000, having multiple streams of data flowing between the computers would fully utilise the trunked ports.

The Summit/400 selects on source and destination IP address. If the data is coming from the Summit/400, having multiple streams of data flowing between the computers would utilise one cable between the Summit/400 and the RS/3000 .